1. Field of the Invention
The present invention relates to switching mode power converter circuits. More particularly, the invention relates to dual mode over-current protection circuit for a switching mode power converter that provides cycle-by-cycle mode over-current control during normal operating conditions and hiccup mode over-current control during short circuit or over-load operating conditions.
2. Description of Related Art
Switching mode DC-to-DC power converters are commonly used in the electronics industry to convert an available direct current (DC) level voltage to another DC level voltage. A switching mode converter provides a regulated DC output voltage to a load by selectively storing energy in an output inductor coupled to the load by switching the flow of current into the output inductor. An isolated forward converter is a particular type of switching mode converter in which an input DC voltage is periodically switched across the primary side of a transformer using one or more power switches, typically provided by MOSFET devices. The transformer provides isolation between the voltage source on the primary side and a load on the secondary side. Two diodes on the secondary side rectify the switched and isolated input voltage, including a forward diode connected in series with secondary winding that conducts current to the load when the input voltage is present across the secondary winding and a free-wheeling diode connected in shunt with the secondary winding that conducts current to the load when the input voltage is not present across the secondary winding.
In order to improve the efficiency of the forward converter circuits, it is known to replace the rectifying diodes with power switches (e.g., MOSFET devices). The operation of the power switches may be controlled so that they are turned on and off in synchronism with the switched input voltage. The control signals applied to the power switches must be synchronized as closely as possible to the current inflection points of the output inductor. This synchronous rectification provides higher efficiency over the foregoing forward converter configuration since the forward voltage drop of the power switches is much lower than that of diodes.
It is known to provide switching mode power converters with specialized circuits that protect against over-current conditions. An over-current condition occurs when the output current delivered to a load exceeds a predetermined limit. The over-current condition can damage the power converter and/or the load. The protection circuits detect an over-current condition and take corrective action to reduce the output current. A first type of current control circuit provides cycle-by-cycle protection by holding the power switch on the primary side in an off state temporarily when the sensed output current reaches an over-current voltage reference. Notably, the power converter is not turned off when the over-current condition is detected. Instead, by keeping the primary side power switch in an off state, the output current will decrease to an acceptable level, whereupon the power converter resumes ordinary operation by periodically activating the primary side power switch. This type of cycle-by-cycle mode current control is advantageous for conditions in which the over-current condition is transient and not particularly severe.
A second type of current control circuit provides hiccup protection by shutting off the power converter entirely when the sensed current signal reaches an over-current limit threshold. This type of hiccup mode current control is advantageous for conditions in which the over-current condition is sustained and presents a significant risk of damage to the power converter and/or load. A drawback of this second type of current control circuit is that it may activate a complete shut down of the power converter at times when the over-current condition is not actually that severe. For example, during a start-up process of a power system in which two or more power converters are operating in parallel, an imbalanced load sharing condition may cause the power converters to become stuck in a cycle of alternatingly activating the hiccup mode current control.
Accordingly, it would be desirable to provide an over-current protection circuit for a switching mode power converter that has the benefits of both cycle-by-cycle and hiccup mode current control.